MANAGED-PRESSURE DRILLING; TECHNIQUES AND OPTIONS ...

M. R. Rohani/Petroleum & Coal 54(1) 24-33, 2012 29Fig. 5. Pressure-gradient profile Fig. 6. Rig up for Pressurized Mud Cap Drilling Operations [12]for pressurized mud cap drillingmethod [16]8. Controlled Mud Cap SystemA newer drilling concept is the controlled mud cap system (CMC). This system is similar tothe pressurized mud-cap system, except that the level of the mud cap is adjusted by a mudpump to better manage the bottom hole pressure. Fig. 7 shows a basic setup of this systemfor a well being drilled in deepwater. A subsea mudlift-pump is connected to the riser by ariser-outlet joint. The outlet joint has high-pressure valves that enable it to isolate the pumpsystem from the riser. The pump is connected to the mud pits by a return and a fill line. Thisallows the pump to increase or decrease the amount of mud in the riser. To determine thelevel of the mud in the riser, pressure sensors are located throughout the riser. The drillingriser is filled with air above the mud cap [17] . The basic concept of this system is to compensatefor ECD and thus manage the BHP.This system also is unique in that it can be operated as either an open or closed system.The first advantage to an open system is that it needs no continuous closure elements totrap pressure in the well. This comes in handy when considerable rig movement can affectthe downhole pressure control. This effect can occur when slips are set to make a pipe connection.With the CMC system, the downhole pressure regime will generally be the same as inconventional drilling except the mud weight may be higher and part of the drilling riser maybe filled with gas. The second advantage with an open system is that a positive riser margincan be designed to be included in this system. With this system the hydrostatic pressure inthe riser at sea level can be designed to equal or be less than seawater pressure. This means apositive riser margin can be added with no overbalance in the well. This positive riser marginmeans that if the riser was to disconnect, the BHP would increase thus improving well control.The third advantage is the CMC system’s ability to handle hydrocarbons. The system operatesas an open system until one of the rams of the surface BOP is closed. Since this system actsas an open system with gas pressure close to ambient, the drilling riser effectively becomesthe hydrocarbon separator. The gas is separated in the riser and the liquids are transportedthrough the pump system up to the rig. Being able to regulate the mud level while thishappens enables fast and accurate changes to the BHP [17] .